The present invention relates to a reproduction method for magneto-optical recording disks, a magneto-optical recording disk driving device, and a magneto-optical recording disk.
In a magneto-optical recording disk, a magnetic thin film thereof is locally heated by laser beam irradiation or other light irradiation to decrease its coercive force. Then, an external magnetic field is applied to the magnetic thin film to invert the direction of magnetization of the irradiated spots or maintain the initial magnetization direction, thereby forming magnetic domains (recorded marks). The direction of magnetization of the magnetic domains is read out by the Kerr or Faraday effect.
In an ordinary magneto-optical recording disk, the available density of reproducible magnetic domains is limited by the spot diameter of the laser beam used for reading, and so it is impossible to read magnetic domains having a diameter smaller than the half of the laser beam spot diameter.
For a magneto-optical recording medium capable of reading magnetic domains having a diameter smaller than the half of laser beam spot diameter, for instance, JP-A 8-7350 discloses a magneto-optical recording medium capable of enlarging recorded magnetic domains. This magneto-optical recording medium comprises a three-layered recording film comprising, in order from an substrate side, a first magnetic layer, a second magnetic layer, and a third magnetic layer which are exchange coupled together. Upon reading, the substrate of the medium is irradiated with a laser beam while a reading magnetic field is applied to the recording film of the medium. The third magnetic layer holds recorded magnetic domains, which are copied by laser beam irradiation to the first, and second magnetic layers. The copied magnetic domains are enlarged by the application of a reading magnetic field in the longitudinal direction of each magnetic layer. The enlarged, copied magnetic domains are read out as is the case with a conventional magneto-optical recording medium. After the reading of the enlarged, copied magnetic domains has finished, an erasing magnetic field opposite in direction to the reading magnetic field is applied to erase off the copied magnetic domains for the next reading of adjacent recorded magnetic domains. By repetition of such a process, it is possible to read minute magnetic domains unreadable so far in the art. Furthermore, this method does not only achieve high resolution upon reading, but also enables output signal strength to be in itself augmented because the magnetic domains are actually enlarged. Hereinafter, the magneto-optical recording medium using such a process for reading will be called a magneto-optical recording medium of the domain enlargement reading type.
For reading the magneto-optical recording medium of the domain enlargement reading type, it is thus required to apply thereto an alternating magnetic field comprising a read bias magnetic field and an erase bias magnetic field.
However, when a modulated magnetic field is applied on an ordinary magneto-optical recording disk for reading purposes, magnetic domains recorded thereon are often erased off. Never until now is it attempted to run an ordinary magneto-optical recording disk and a magneto-optical recording disk of the domain enlargement reading type on the same driving device for reading purposes.